High Voltage Power Supply for Static Neutralizers

ABSTRACT

A high voltage power supply for a static neutralizer is disclosed. The high voltage power supply includes a resonant converter and a load with an emitter module having an emitter, reference electrode, and a capacitance value. The resonant converter is disposed to have a resonant frequency and an output coupled to the load. The resonant converter generates an output waveform with an amplitude sufficient for generating to ions by corona discharge when the load receives the output waveform. The load is predominantly capacitive when the resonant converter is operating at the resonant frequency.

BACKGROUND

(1) Technical Field

The present invention relates to high voltage power supplies for staticneutralizers. More particularly, the present invention relates to highvoltage power supplies that employ a resonant converter having a highlyefficient circuit design. This resonant converter is suitable fordriving loads with relatively high capacitance, such as emitters used instatic neutralizers for generating bipolar ions by corona discharge, forenabling a compact, small foot-print implementation of these powersupplies, or both.

(2) Background Art

Static neutralizers are commonly employed by the electronics industry toreduce or eliminate electro-static charge from static-sensitivecomponents or equivalent charged objects. Static neutralizers aredesigned to eliminate or minimize static charge from these chargedobjects by generating bipolar air or, in some instances gas ions, anddelivering these air or gas ions to the charged object. Staticneutralizers employ a set of emitters, sometimes referred to as ionizingelectrodes, corona electrodes, or corona filaments or wires. Eachemitter is disposed to have a shape suitable for generating ions bycorona discharge. A common emitter shape includes a long thincylindrical shape, such as a thin wire or filament, or an end portionhaving a small tip radius or a sharp point. These emitters are sometimeshoused in an emitter module or cell that may include one or moreconducting surfaces coupled to a reference potential, such as earthground or circuit ground. These conducting surfaces are commonlyreferred to collectively as a “reference electrode.”

Generating ions by corona discharge requires applying a relatively highelectrical potential to at least one emitter in order to create largevoltage gradients at points of high curvature on the emitter surface.When a sufficiently large voltage gradient exists on its surface, apositively charged electrode produces a cloud of positive ions bycollecting electrons from nearby air molecules. Similarly, a negativelycharged electrode produces a cloud of negative ions by transferringelectrons onto nearby air molecules. Collectively, these positively andnegatively charged ions are sometimes referred to as a bipolar ion cloudand are considered useful for static neutralization since the bipolarion cloud contains a group of ions that have a mix of polarities thatwill maximize charge neutralization for a charged object selected forneutralization. The proportion of negative and positively charged ionsmay change depending on the environment conditions in which the staticneutralizer is used.

To create a mix of ions having positive and negative charges, thesestatic neutralizers may use an alternating high voltage waveform.Because opposite electrical charges attract, negatively charged ions aredrawn to positively charged surfaces while positively charged ions aredrawn to negatively charged surfaces. Once these ions reach a chargedsurface, the ions compensate for an excess of positive or negativecharges on the surface, diminishing and thereby “neutralizing” staticcharge on the surface and reducing the associated hazards with thesestatic charges.

These emitters, emitter modules or both exhibit an impedancecharacteristic that includes a relatively high capacitance which oftenexceeds 100 pF, requiring a high voltage power supply capable of drivingthe waveform at a frequency and amplitude suitable for creating ions bycorona discharge. Since the corona voltage or waveform amplituderequired to create ions by corona discharge is high, this power supplymust have sufficient voltage and current driving capacity, which usuallyrequires a large and bulky transformer. Besides the expense associatedwith using a large transformer, the bulk of the transformer limitsplacement versatility of the emitter(s), emitter module or both.

A class of circuits known variously as resonant converters or resonantinverters is commonly used to generate high voltage sine waves from lowvoltage DC inputs. This class of circuits is frequently used inelectronic ballasts that excite Cold Cathode Fluorescent Lights andCompact Fluorescent Lights, which may respectively be referred to as“CCFL” and “CFL”. One of these topologies is the push-pull version ofthe current fed Class-D parallel resonant converter, sometimes calledthe Baxandall oscillator. An example of this architecture is shown inFIG. 1.

Using a resonant converter 8 of the type disclosed in FIG. 1 to avoidthe use of a large and bulky transformer in a high voltage power supplythat is suitable for use in a static neutralizer would not be obvious totry for many reasons. Resonant converters currently used in electronicballasts that excite CCFLs normally only provide a strike voltage ofapproximately 2500 volts peak for a fraction of a second and thenproduce a sinusoidal waveform with peak amplitude that is typically lessthan 1000 volts during continuous operation after ignition. Theamplitude of the exciting waveform typically used to create ions bycorona discharge, in contrast, is higher, typically in the range of 3500volts peak to 7500 volts peak. In addition, the capacitive load of aCCFL tube before striking is typically only around 10 pF while thecapacitive load presented by an ionizer is often higher and may exceed100 pF in some instances. The high frequency power supply driving theemitters in a static neutralizer frequently operate continuously forrelatively long periods of time, while generating much higher outputvoltages and output currents than required of the power supplies thatdrive CCFLs. These factors generally make the power supplies for highfrequency static neutralizers much larger, more expensive, and moredifficult to design than the power supplies that drive CCFLs.Furthermore, the need to produce high frequency, high voltage waveformsin a confined space leads to a host of problems with conventionalresonant converter designs as a result of spurious oscillation modesthat arise from parasitic circuit elements, as well as problemsconcerning the reduced efficiency of compact designs and relatedproblems associated with reliability and thermal management.

Consequently, there is a need for a new type of a high voltage powersupply that is efficient yet capable of driving a high frequency, highvoltage waveform onto at least one emitter so that ions are created bycorona discharge. Moreover, there is a need for a high voltage powersupply this not only efficient but is also low cost and suitable forcompact, small foot print implementations.

SUMMARY

A high voltage power supply for a static neutralizer is disclosed. Thehigh voltage power supply includes a resonant converter and a load withan emitter module having an emitter, reference electrode and acapacitance value. The resonant converter is disposed to have a resonantfrequency and an output coupled to the load. The resonant convertergenerates an output waveform with an amplitude sufficient for generatingions by corona discharge when the load receives the output waveform. Theload is predominantly capacitive when the resonant converter isoperating at the resonant frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Class D parallel resonant converter circuit.

FIG. 2 illustrates a high voltage power supply that uses a resonantconverter in accordance with one embodiment of the present invention.

FIG. 3A illustrates a resonant converter that includes an inductance setwithin a resonant tank circuit in accordance with another embodiment ofthe present invention.

FIG. 3B illustrates a generalized electrical model of an emitter andemitter module that may be used with the resonant converter disclosed inFIG. 3A.

FIG. 4 illustrates a resonant converter that attenuates or reducesspurious oscillations in accordance with yet another embodiment of thepresent invention.

FIG. 5 illustrates a resonant converter that operates with reducedlosses from parasitic capacitance in accordance with a furtherembodiment of the present invention;

FIG. 6 illustrates a resonant converter that includes a control circuitin accordance with yet another further embodiment of the presentinvention;

FIG. 7 illustrates a resonant converter that generates a feedback signalby sensing output voltage in accordance with a further embodiment of thepresent invention; and

FIG. 8 illustrates a resonant converter that includes a full bridgedrive circuit in accordance with yet another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, for purposes of explanation,numerous specific details are set forth to provide a thoroughunderstanding of the various embodiments of the present invention. Thoseof ordinary skill in the art will realize that these various embodimentsof the present invention are illustrative only and are not intended tobe limiting in any way. Other embodiments of the present invention willreadily suggest themselves to such skilled persons having benefit of theherein disclosure.

In addition, for clarity purposes, not all of the routine features ofthe embodiments described herein are shown or described. It isappreciated that in the development of any such actual implementation,numerous implementation-specific decisions must be made to achieve thedeveloper's specific goals. These specific goals will vary from oneimplementation to another and from one developer to another. Moreover,it will be appreciated that such a development effort might be complexand time-consuming but would nevertheless be a routine engineeringundertaking for those of ordinary skill in the art having the benefit ofthe herein disclosure.

Referring now to FIG. 2, a high voltage power supply 10 that includes aresonant converter 12, and a DC power supply 14 that provides excitationenergy, which may be in the form of a low voltage 15 and a current 16,to resonant converter 12. Resonant converter 12 includes and isconfigured to resonate with a load 18 that includes at least one emitter20, which renders power supply 10 suitable for use with a staticneutralizer 22. Static neutralizer 22 may include a reference electrode23 and an emitter module 24 that may be used to house emitter 20. Load18 is predominantly capacitive at the resonant frequency of resonantconverter 12, exhibiting a quality factor or “Q factor” that maytypically be in an approximate range from 2 to 1000 at this frequency,depending on the dielectric properties of the insulating materials usedto construct emitter 20 and emitter module 24 and also on the level ofcorona discharge produced by emitter 20 during operation. In the exampleshown in FIG. 2, load 18 not only includes emitter 20 and emitter module24 but also is modified to further include an ion balance circuit 26.

During operation of static neutralizer 22, emitter 20 receives a highvoltage waveform, named “output waveform”, 28 from a transformer outputor “waveform output” 27 of transformer 36. Output waveform 28 isapproximately sinusoidal and has an amplitude, a frequency and anoscillation period. Output waveform 28 may be in any form suitable forgenerating a set of ions by corona discharge when output waveform 28 isapplied to emitter 20. The frequency may range approximately from 2 kHzto 100 kHz and is not intended to be limiting in any way but may bedetermined as required to effect static charge neutralization from acharged object (not shown) selected for neutralization, named the“target object”. The set of ions generated includes ions of bothpolarities, sometimes referred to as a “bipolar ion cloud”.

DC balancing circuit 26, such as a capacitor (not shown) having a valuethat is typically several times larger than the capacitance exhibited byemitter 20, may be provided as part of static neutralizer 22. DCbalancing circuit 26 provides a DC offset to output waveform 28 so thatan approximate balance of positive and negative ions, named “ionbalance”, reach the surface of the target object. Since ion balance willvary according to environmental conditions encountered by and thephysical implementation of static neutralizer 22, the value of thiscapacitor is not intended to limit the embodiment shown in any way.

Resonant converter 12, which may also be referred to as a “powerinverter” or “inverter”, generates output waveform 28 in response toreceiving excitation energy from a suitable power source, such as DCpower supply 14. DC power supply 14 may be a voltage source or a currentsource power supply circuit, depending on the requirements of theapplication and thus, the implementation of DC power supply 14 is notintended to limit the present invention in any way. DC power supply 14is coupled to ground 32, and includes an output 30 that may be coupledto resonant converter 12 through an isolation circuit 34. Isolationcircuit 34 permits excitation energy, which may be in the form of lowvoltage 15 and current 16, to flow from DC power supply 14 to atransformer 36, and limits DC power supply 14 from acting as anotherload on resonant converter 12 by reducing the flow of high frequencycurrent between transformer 36 and DC power supply 14 during operation.Low voltage 15 has a value in an approximate range from 5 to 100 Volts,while current 16 has a value in an approximate range from 0.1 to 3 Amps.These values are not intended to be limiting in any way. The term“ground” is not intended to be limited to earth ground but may be areference potential used by resonant converter 12 as its signal ground.

Besides transformer 36, resonant converter 12 may further include acommutation circuit 46 and a sensing circuit 42, while transformer 36includes a primary winding 38, secondary winding 40 and core 44.Commutation circuit 46 includes a capacitor 48, a bias resistor 50 and aswitching circuit 52. Sensing circuit 42 may be implemented by using afeedback winding 43 on the same core used by secondary winding 40 intransformer 36, such as core 44.

Primary winding 38 includes a center tap 54 and legs 56 and 58. Legs 56and 58 each have a pair of ends, such as ends 60 a and 60 b and ends 62a and 62 b, respectively. To obtain closely matched characteristics,legs 56 and 58 may be wound together, typically using either bifilarwire or closely placed parallel strands. Ends 60 b and 62 a form centertap 54, which is disposed to receive a current from output 30 throughisolation circuit 34.

In the example shown, isolation circuit 34 may be implemented by usingan AC choke coil (not shown) that has a value approximately within anapproximate range from 50 to 1000 μH. Using an AC choke coil having avalue within this range is not intended to limit the example shown inany way, and other values may be used as long as the value selectedenables the AC choke coil to isolate resonant converter 12 from DC powersupply 14 and allows a current from output 30 to reach primary winding38 via center tap 54. Using isolation circuit 34 renders resonantconverter 12 as a “current-fed” circuit since isolation circuit 34forces a current into primary winding 38. Transformer 36 may be of thebobbin-wound design variety although this implementation detail is notintended to be limiting in any way.

Switching circuit 52 may be implemented using a pair of electronicswitching elements, named “switch elements”, 66 and 68. In the exampleshown, switch elements 66 and 68 are implemented in the form of bipolarNPN transistors 76 and 78, respectively. The use of bipolar NPNtransistors is not intended to be limiting in any way, and one ofordinary skill in the art having the benefit of the herein disclosurewould readily recognize that any suitable switch element may beemployed, such as a MOSFET, JFET, IGBT, MCT, thyristor, opto-isolator orequivalent switch element.

Ends 60 a and 62 b respectively connect to the collectors of bipolar NPNtransistors 76 and 78, while the emitter terminals of transistors 76 and78 connect to ground 32. Ends 72 and 74 of feedback winding 43 connectto the base terminals of transistors 76 and 78, respectively.

Secondary winding 40 is coupled magnetically to feedback winding 43 andprimary winding 38 through core 44. Core 44 may be implemented using alow-loss ferrite material, powdered iron material or any equivalentmaterial that will permit transformer 36 to couple magnetically feedbackwinding 42 with secondary winding 40. Secondary winding 40 is coupled toemitter 20, which is disposed to have a high Q factor at the resonantfrequency of resonant converter 12. Secondary winding 40 contains alarge number of turns relative to primary winding 40 to provide the stepup ratio needed to convert a voltage provided by DC power supply 14 atoutput 30 into a high voltage sinusoidal waveform across secondarywinding 40 during operation of resonant converter 12. Capacitor 48connects across to ends 60 a and 62 b, which provides a shaping functionand ensures zero voltage switching of transistors 76 and 78. Theplacement of capacitor 48 within commutation circuit 46 permitscapacitor 48 to also minimize voltage spikes during commutation oftransistors 76 and 78 that result from leakage inductance in series withlegs 56 and 58. In an alternative embodiment of the example shown inFIG. 2, capacitor 48 may be omitted if the shaping function, zerovoltage switching, minimization of voltage spikes or any of combinationof these features is not required.

The total reactance of elements that is reflected either to primarywinding 38 or to secondary winding 40 define, for resonant converter 12,an equivalent resonant tank circuit that has a resonant frequency basedon the total reactance of these elements. Resonant converter 12maintains an oscillation at this resonant frequency. These elementsinclude the magnetizing inductance of transformer 36, the leakageinductance of primary winding 38 and secondary winding 40, theinter-winding, intra-winding, and winding-to-core parasitic capacitancein primary winding 38 and secondary winding 40, the capacitance ofcapacitor 48, and the total capacitance of load 18.

In the example in FIG. 2, the capacitance of load 18 includes thecapacitance of emitter 20, emitter module 24 or both, depending on thedesign of emitter 20 and emitter module 24. The capacitance of load 18may also include additional elements, such as those in ion balancecircuit 26, that are part of the load seen by secondary winding 40 andthereby influence the value of the resonant frequency of resonantconverter 12. The capacitance of these additional elements plus thecapacitance of load 2 are collectively referred to as the “outputcapacitance” of secondary winding 40.

If an AC choke coil is used to implement isolation circuit 34, the ACchoke coil is configured to have high impedance when operating resonantconverter 12 at the resonant frequency of its equivalent tank circuit.This configuration isolates the equivalent tank circuit from DC powersupply 14 and limits DC power supply 14 from acting as a load onresonant converter 12. Isolating the equivalent tank circuit from DCpower supply 14 when operating at the resonant frequency, allows theresonant tank circuit to have a high Q factor, and results in reducedlosses in resonant converter 12 and improved purity of the output waveform, such as output waveform 28.

Switching circuit 52 creates and maintains the oscillation in resonantconverter 12. In the embodiment shown, switch elements 66 and 68alternately reverse roles as one switch element turns on while the otherswitch element turns off. This switching behavior, commonly referred toas “commutation”, is caused by the generation of a feedback signal 70 bysensing circuit 42. In the example shown in FIG. 2, feedback signal 70is in the form of a differential voltage waveform and is transmitted bysensing circuit 42 to the base terminals of bipolar NPN transistors 76and 78 through ends 72 and 74, respectively. Since in the example shownin FIG. 2, sensing circuit 42 is implemented using feedback winding 43,feedback signal 70 is approximately equal to the oscillating waveformvoltage formed across load 18 scaled down by the step down ratio betweensecondary winding 40 and feedback winding 43. The step down ratio usedin the example shown in FIG. 2 may be selected to generate a feedbacksignal 70 of an amplitude that will effect proper commutation withoutapplying damaging voltages to switch elements 66 and 68. Typically apeak-to-peak voltage of around 4 volts is selected if switch elements 66and 68 are bipolar NPN transistors. The resulting feedback signal 70 issmaller than but approximately proportional to the oscillating waveformvoltage formed across load 18. Using feedback winding 43 to generatefeedback signal 70 is not intended to be limiting in any way, but anyequivalent sensing circuit may be used that can generate a feedbacksignal that can be used to trigger the commutation of switch elements 66and 68.

To create and maintain an oscillation within resonant converter 12, biasresistor 50 supplies sufficient current to the base terminal of eithertransistor 76 or transistor 78, which causes one of these transistors toturn fully-on, while voltage induced into feedback winding 42 by theoscillation keeps the other transistor turned-off. The fully-ontransistor allows current to flow from DC power supply 14 throughisolation circuit 34, through either leg 56 or 58 and to ground 32 viathe collector and emitter terminals of the transistor that is fully-on.

For example, during a phase within oscillation cycle, bias resistor 50supplies sufficient current to the base terminal of transistor 76,activating transistor 76. The voltage induced into feedback winding 43generates feedback signal 70, keeping transistor 78 off. Activatedtransistor 76 allows current to flow from DC power supply 14 throughisolation circuit 34, through leg 56 and to ground via the collector andemitter terminals of transistor 76.

In another example, during another phase within the oscillation cycle,bias resistor 50 supplies sufficient current to the base terminal oftransistor 78 through feedback winding 43, activating transistor 78.This oscillation cycle induces a voltage into feedback winding 43,generating feedback signal 70 and keeping transistor 76 off. Activatedtransistor 78 allows current to flow from DC power supply 14 throughisolation circuit 34, through leg 58 and to ground via the collector andemitter terminals of transistor 78.

During each cycle of oscillation, energy is transformed from electricalenergy to magnetic energy and back again to electrical energy. Themagnetic energy is stored in the magnetic field associated with core 44of transformer 36, usually principally in one or more gaps in themagnetic circuit of transformer 36. The electrical energy is stored inthe capacitive elements of resonant converter 12 including the outputcapacitance, capacitor 48, and in the various parasitic capacitiveelements of transformer 36. When the voltage across load 18 is nearzero, most of the energy in resonant converter 12 is stored in themagnetic field associated with core 44 of transformer 36. When thevoltage across load 18 is near a positive or negative peak, most of theenergy in resonant converter 12 is stored in the capacitive elements ofresonant converter 12.

Because core 44 of resonant converter 12 stores essentially all of theenergy that it transfers each cycle to the capacitive elements ofresonant converter 12, the physical size of core 44 typically increasesas the peak output voltage or the output capacitance increases. The sizeof core 44 must also increase in this case because the winding window ofcore 44 is typically required to be large enough to accommodate theheavier wire that would be selected for secondary winding 40 in orderfor secondary winding 40 to carry the larger currents that pass throughit as the peak output voltage and the output capacitance increases.

In accordance with an alternative embodiment of the present invention,the embodiment disclosed in FIG. 2 may include integrating high voltagepower supply 10 with emitter module 24. The low profile characteristicof high voltage power supply 10 enables this approach novel since powersupply 10 not only includes a resonant converter, such as resonantconverter 12, that is suitable for providing a waveform that is suitablefor generating ions by corona discharge that are effective instatic-charge neutralization but also suitable for use as a low-profilesolution that can be integrated with the emitter module 24.

Turning now to FIG. 3A, a novel resonant converter 84 is disclosed inaccordance with yet another embodiment of the present invention.Resonant converter 84 may be used as part of a compact, small foot-printpower supply 82, and generates a high voltage output waveform 156 thatis suitable for driving a load 110 that is predominantly capacitive andthat has a relatively high Q factor, when resonant converter 84 operatesat its resonant frequency. For example, load 110 may include at leastone emitter 96, rendering power supply 82, resonant converter 84 or bothhighly suitable for use with a static neutralizer 80. In an alternateimplementation, load 110 may further include an ion balancing circuit108 and an emitter module 98, as shown. Emitter module 98 may include areference electrode 99. Emitter module 98 may be used to house emitter96 and may be included as part of static neutralizer 80.

Resonant converter 84 includes a transformer 86 coupled to an inductanceset 88 that includes at least one inductive element, such as inductiveelements 90, 92 and 94. Voltage stresses and energy storage requirementsthat would otherwise be carried by transformer 86 are insteaddistributed across the inductive elements included as part of inductanceset 88. This solution reduces loading from parasitic capacitance in theinternal circuit components of transformer 86 and reduces electricalstress within these components. Consequently, a power supply, such aspower supply 82, that employs resonant converter 84 can be madephysically smaller, more efficient, more reliable, easier to optimize,and less expensive to manufacture than conventional designs.

Although FIG. 3A discloses using resonant converter 84 with staticneutralizer 80, the novel nature of resonant converter 84 renders ithighly suitable for use in other applications that require a compact,efficient, and low cost high voltage waveform generator, such asapplications that include providing power for CCFL backlighting in smallform factor devices, such as LCD monitors, as well as applications thatprovide power in compact fluorescent lights. Resonant converter 84 mayalso be used in power supplies configured for use with air purificationsystems and other power supplies that provide high voltage AC or DCpower.

Power supply 82 includes a DC power supply 100 and resonant converter84, as well as an isolation circuit 102 coupled to an output 104 of DCpower supply 100 and a center tap 106 of transformer 86. DC power supply100 provides excitation energy, which may be in the form of a lowvoltage 105 and a current 107, to resonant converter 84 through output104. Emitter 96 and emitter module 98 may be generalized as anelectrical load 111 that has a resistance 115 in parallel with acapacitance 112 coupled to a resistance 114 in series, as illustrated inFIG. 3B. Resistance 115 models losses associated with corona dischargefrom emitter 96, while resistance 114 models conductive and dielectriclosses in emitter 96 and emitter module 98. Capacitance 112 models thecapacitive load provided by emitter 96 and emitter module 98.

One ordinary of skill in the art after receiving the benefit of theherein disclosure would readily recognize that the circuit designdisclosed in FIG. 3A is similar to the circuit design disclosed in FIG.2 above, except for the inclusion of inductance set 88. DC Power supply100, isolation circuit 102 and commutation circuit 116 may beimplemented to have substantially the same function as DC power supply14, isolation circuit 34 and commutation circuit 46 that were previouslydisclosed above with respect to FIG. 2.

Besides transformer 86 and inductance set 88, resonant converter 84further includes a commutation circuit 116 and a sensing circuit 118,while transformer 86 also includes a primary winding 120, secondarywinding 122 and core 124. Commutation circuit 116 includes a capacitor126, a bias resistor 128 and a switching circuit 130. Sensing circuit118 is implemented using a feedback winding 132 on the same core used byprimary winding 120 and secondary winding 122 in transformer 86, such ascore 124. Primary winding 120 includes center tap 106 and legs 134 and136. Legs 134 and 136 each have a pair of ends, such as ends 138 a and138 b and ends 140 a and 140 b, respectively. To obtain closely matchedcharacteristics, legs 134 and 136 may be wound together, typically usingeither bifilar wire or closely placed parallel strands. Ends 138 b and140 a form center tap 106, which is disposed to receive excitationenergy, such as low voltage 105 and current 107, from output 104.

During operation of resonant converter 84, sensing circuit 118 providesa feedback signal 142 to a switching circuit 130, which causes a pair ofswitch elements 144 and 146, such as bipolar NPN transistors 148 and150, to switch-on and switch-off in an alternating fashion and whichcreates and sustains an oscillation in a secondary tank circuit 152.Bias current for transistor 148 flows through bias resistor 128, whilebias current for transistor 150 flows through bias resistor 128 andthrough sensing circuit 118, such as feedback winding 132. Base terminalcurrent through transistor 148 or 150 allows current to flow from DCpower supply 100, through isolation circuit 102, through leg 134 or 136of center-tapped primary winding 120 to ground 154, inducing a magneticfield into core 124. This magnetic field induces voltage into secondarywinding 122, causing current to flow in secondary tank circuit 152. Inthe example shown, secondary tank circuit 152 includes secondary winding122, inductance set 88 and load 110. The resonant frequency of secondarytank circuit 152 essentially determines the operating frequency ofresonant converter 84.

During steady state operation of resonant converter 84, a sinusoidalcurrent oscillates at resonant frequency of secondary tank circuit 152.This oscillating sinusoidal current induces a leading voltage acrosssecondary winding 122 and inductance set 88 in proportion to theirinductive reactance at the oscillation frequency. The voltage appearingacross inductance set 88 is in phase with the voltage across secondarywinding 122. The sum of these voltages appears across load 110.

If ion balance circuit 108 is implemented as a capacitor having a fixedcapacitance, the value of capacitor 108 is usually selected to beseveral times larger than the capacitance of emitter 96, emitter module98, or both. Consequently, capacitor 108 has little effect on theoperating frequency of resonant converter 84 or on the AC voltageappearing across load 110. Capacitor 108 may be omitted if resonantconverter is used in applications that do not include an ion emitter,such as emitter 96.

Including inductance set 88 as part of secondary tank circuit 152 lowersthe resonant frequency of secondary tank circuit 152 by addinginductance. Including inductance set 88 also increases the outputvoltage, named output waveform 156, appearing across load 110 by theratio of total apparent inductance of the secondary tank circuit 152 tothe apparent inductance of secondary winding 122. Apparent inductance isdefined as the frequency-dependent inductance measure that adjusts forthe effect that capacitor 126 has on the reactance of secondary winding122 at the operating frequency of resonant converter 84. The inclusionof inductance set 88 enables large improvements to the size, cost,reliability, and efficiency of resonant converter 84 when compared toconventional resonant converter designs, such as the circuit designdisclosed for resonant converter 8 in FIG. 1.

The example in FIG. 3 discloses that the energy stored in each inductorin the secondary tank circuit is proportional to its inductance. Byallowing the inductance of secondary winding 122 to be reduced,inductance set 88 reduces the amount of energy stored in the magneticcircuit of transformer 86 and enables transformer 86 to be wound using asmaller core, a core a smaller magnetic gap, and windings containing asmaller number of turns than would otherwise be required without the useof inductance set 88.

Reducing transformer size, in turn, reduces the effective straycapacitance appearing across secondary winding 122. Stray capacitancetypically arises as a result of inter-winding capacitance, intra-windingcapacitance, and winding to core capacitance. This stray capacitancetends to drop linearly as the linear dimensions of the transformershrink. The process of reducing both the inductance of secondary winding122 and the effective stray capacitance across secondary winding 122 hasthe effect of raising the self-resonant frequency of transformer 86,allowing the transformer to operate efficiently at higher frequenciesthan would otherwise be possible.

Feedback winding 132 and primary winding 120 are both near groundpotential. Transformer 86 may be implemented by using a bobbin-wounddesign that includes a sectioned bobbin, while core 124 may beimplemented using a gapped ferrite or powdered iron core. Using asectioned bobbin and a gapped ferrite or powdered iron core is notintended to be limiting in any way since other core and winding typesmay be selected depending on the application.

To minimize cost and to simplify the sourcing of materials, inductanceset 88 may be constructed using the same bobbin and core design used fortransformer 86, though the magnetic gap selected for transformer 86 maydiffer from that selected for each inductance element in inductance set88. For example, if an inductance element, such as inductance element90, 92 or 94, is implemented using an inductor, the inductor may bedisposed to include a single winding with no additional windingselectrically loading its core. Each inductor used in inductance set 88may be physically separated by distances sufficient to allow onlyminimal electric field interaction with adjacent elements. Consequently,the electrical potential of the core of each inductor is free to floatto the average voltage along its winding. This minimizes voltage stressbetween the windings and the cores and also minimizes losses associatedwith the displacement currents that flow in the coil form between thewinding and the magnetic core of each inductor in inductance set 88.

The inclusion of isolation circuit 102 with power supply 82 is anoptional implementation of the example shown in FIG. 3A. Isolationcircuit 102 essentially isolates secondary tank circuit 152 from loadingby DC power supply 100 during the operation of resonant converter 84,while also permitting DC power supply 100 to provide excitation energyto resonant converter 84. Isolation circuit 102 may be implemented usingan AC choke coil that has an inductance value of approximately between50 and 1000 μH although this range is not intended to be limiting in anyway. Any AC choke coil value may be used that is sufficient to isolatesecondary tank circuit 152, while permitting a current, or equivalentexcitation energy, to reach resonant converter 84. For example, AC chokecoil may have a value that is larger than the leakage and magnetizinginductance of each leg, such as legs 134 and leg 136, of primary winding120.

The sudden switching of current between the two legs of primary winding120, such as legs 134 and 136, caused by commutation of switch elements144 and 146, such as transistors 148 and 150, may generate large voltagetransients unless snubbing components are available to absorb thesetransients. In resonant converter 12 of FIG. 2, this function isprovided by transformer 36 in conjunction with load 18 and capacitor 48.In the example shown in FIG. 3, however, load 110 is isolated fromtransformer 86 by inductance set 88, so the full commutation transientis absorbed by capacitor 126 through transformer 86. To enhance furtherthe attenuation of these transients, particularly those associated withleakage inductance in primary winding 134, additional snubbing circuitrycomprising components, such as resistors, capacitors, transient voltagesuppressors, metal oxide varistors, Zener diodes, spark gaps, orcombinations of these components may be added (not shown). Thesecomponents may be connected between the emitter and collector terminalsof transistors 148 and 150, respectively, and also sometimes acrossisolation circuit 102.

During steady state operation of resonant converter 84, commutation ofswitch elements 144 and 146 excite currents that circulate in theresonant tank circuit, named “primary tank circuit,” that includesprimary winding 120 and capacitor 126. The value of capacitor 126 andthe sum of the leakage and magnetizing inductance of primary winding 120determine the frequency of the related resonant mode. In most cases, theefficiency of resonant converter 84 peaks when a value of capacitor 126is selected to place the resonant mode of the primary tank circuit at aneven harmonic of the resonance in secondary tank circuit 152, which isthe operating frequency of resonant converter 84. This selection for thevalue of capacitor 126 minimizes switching losses by causing transistors148 and 150 to commute when the voltage across primary winding 120 isnear zero volts. In most cases, the best efficiency and most reliablebehavior of resonant converter 84 is obtained when the resonantfrequency of primary tank circuit is placed near the second or fourthharmonic of the resonance frequency of secondary tank circuit 152. Inpractice, the value of capacitor 126 is not critical and reasonablyefficient operation can be obtained even when this value varies over arelatively wide range and this value typically must be adjustedexperimentally to obtain maximum circuit efficiency. For example,capacitor 126 may have a value within an approximate range from 50 nF to120 nF. These values are not intended to be limiting in any way and areprovided as an example implementation of a resonant converter that candrive a load within an approximate range from 70 to 100 pF at a resonantfrequency within an approximate range from 10 to 20 kHz, assuming thatlow voltage 105 is around 10 volts, that resonant converter 84 includestransformer 86, inductive elements 90, 92, and 93 as shown in FIG. 3,and that these are all wound on cores of similar size.

When a resonant frequency of primary tank circuit is selected to be nearthe second harmonic of the operating frequency of resonant converter 84,the initial design and analysis of resonant convert 84 is relativelysimple. In most cases, primary winding 120 and secondary winding 122 arerelatively tightly coupled so that leakage inductance in these windingscan be ignored for first-order calculations. If capacitor 126 resonateswith the magnetizing inductance at twice the operating frequency, itscapacitive reactance at the operating frequency of resonant converter 84will be four times the inductive reactance at the operating frequency.This has the effect of increasing the inductive reactance seen acrosssecondary winding 122 by a factor of 4/3 at the operating frequencyabove that associated with the total inductance of secondary winding 122when primary winding 120 is not connected to any circuitry.

Using the components, values and ranges for the elements disclosedherein is not intended to be limiting in any way. One of ordinary skillin the art having the benefit of the herein disclosure would readilyrecognize that the selection of components and their values is affectedby many tradeoffs involving cost, performance, reliability, and desiredform factor. It is convenient for purposes of illustration, however, tomake some simplifying assumptions, none of which are generally requiredby the various embodiments of the present invention. In the designexample presented below, cores and sectioned bobbins of the same sizeand type are used to implement transformer 86 and each inductor ininductance set 88, such as inductors 90, 92, and 94. The magnetic fluxdensity in core 124 is selected to approximately match the magnetic fluxlevels in the cores of each inductor in inductance set 88 during steadystate operation of resonant converter 84. The same wire size and thesame number of windings per bobbin section are used for the windings ofeach inductor in inductance set 88 and for secondary winding 122.

The total inductance of a secondary tank circuit, such as secondary tankcircuit 152, that will be implemented for a resonant converter, such asresonant converter 84, that uses an inductance set, such as inductanceset 88, may be calculated by using the following equation.

$L_{R} = \frac{1}{\left( {2\pi \; F} \right)^{2}C_{R}}$

where “L_(R)” is the total inductance of the secondary tank circuit,“C_(R)” is the capacitance of the load that will be driven by theresonant converter, and “F” is the desired operating frequency of theresonant converter. The desired frequency F may be defined by theconditions necessary to optimize bipolar ion generation by staticneutralizer 80, the capacitance of load 110, as well as the operatingenvironmental conditions to which static neutralizer 80 is exposed. Forexample, frequency F may be within an approximate range from 2 to 100kHz although in the examples shown herein frequency F is within anapproximate range from 10 to 20 kHz.

Since in this example, resonant converter 84 is designed for use with astatic neutralizer, such as static neutralizer 80, C_(R) represents thecapacitance of the emitter, emitter module or both used by the staticneutralizer, such as emitter 96, emitter module 98 or both respectively,while the desired operating frequency F reflects the frequency of theoutput waveform that will be used to create ions by corona dischargewhen the waveform is applied to emitter 96.

The inductance of each inductor in inductance set 88 may then becalculated by selecting an inductance equal to the fraction of totalinductance L_(R) corresponding to the fraction of the total number ofsecondary windings wound on each inductor of the inductance set. Forinstance, if inductors 90, 92 and 94 are configured to have six windingsections, and if transformer 86 includes four secondary winding sectionsand two sections devoted to primary winding 120 and feedback winding132, then secondary tank circuit 152 will include a total of 22 windingsections and the core of each inductor in inductance set 88 may begapped to have an inductance L_(i) set by the following equation.

L _(i) =L _(R)*6/22.

The core of the transformer that will be used for secondary winding 122,such as core 124 and transformer 86, respectively, may be gapped so thatthe secondary winding 122 has a winding inductance that is calculatedusing the following equation:

L _(2w) =L _(R)*(4/22)*(3/4)=L _(R)*3/22

where L_(2w) is the inductance of the secondary winding and the factor3/4 compensates for the effect that capacitor 126 has on the effectivesecondary inductance of transformer 86 when resonant converter 84 isoperating at its operating frequency F.

If the resonant frequency of the primary tank circuit is selected to besomething other than the second harmonic of the operating frequency ofthe resonant circuit, minor adjustments to the above calculation may berequired. For instance, if the resonant frequency of the primary thecircuit is selected to be the fourth harmonic of the operatingfrequency, capacitor 126 causes the apparent inductance of secondarywinding 122 to increase by the ratio 16/15 instead of the ratio 4/3. Forthis case, and with reference to the example above, the core 124 wouldbe gapped so that secondary winding 122 has an inductance L_(2w) that iscalculated by using the following equation:

L _(2w) =L _(R)*(4/22)*(15/16)=L _(R)*15/88

where L_(2w) is the inductance of the secondary winding and the factor15/16 compensates for the effect that capacitor 126 has on the effectivesecondary inductance of transformer 86 when resonant converter 84 isoperating at its operating frequency.

The wire size, number of turns per section, core size, core material,coil form material, and wire insulation material may be selected basedon tradeoffs between core loss, copper loss, magnetic leakage,dielectric losses, desired operating temperature, and otherconsiderations that are known to one of ordinary skill in the art havingthe benefit of the herein disclosure.

The approach above may further include computing the number of turnsrequired for primary winding 120 by using the following relationships.During steady state operation of resonant converter 84, the outputwaveform amplitude across load 110 is determined principally by thevalue of low voltage 105, by the ratio of the number of turns used byprimary winding 120 and secondary winding 122, named “transformer turnsratio”, and by the ratio between the apparent inductance of secondarywinding 122 and the inductance of inductance set 88. Because of magneticcoupling through core 124, approximately identical waveform shapesappear across secondary winding 122, primary winding 120 and feedbackwinding 132. The voltage present on center tap 106 is full waverectified by the commuting action of switch elements 144 and 146, suchas transistors 148 and 150, and has an amplitude equal to one half thatof the peak voltage across primary winding 120. Because inductors passDC voltages, the DC level on each side of isolation circuit 102 isapproximately equal if it is implemented using an AC choke coil.Consequently, the average or DC voltage level at center tap 106 isapproximately equal to the voltage output of DC power supply 100, suchas low voltage 105

It is well known that the average or DC component of a full-waverectified sine wave is equal to 2/π times the peak value of that sinewave. The peak value of the sine wave voltage across primary winding 120is therefore approximately equal to π times the voltage 105 supplied byDC power supply 100. The peak voltage of the sine wave at the operatingfrequency across secondary winding 122 is equal to approximately thisvalue multiplied by the transformer turns ratio between primary winding134 and secondary winding 132. The output waveform across load 110 isapproximately equal to the peak voltage across secondary winding 122multiplied by the ratio of the total inductance of secondary tankcircuit 152 divided by the apparent inductance of secondary winding 122at the operating frequency of resonant converter 84.

Although the above approach teaches relationships that would be founduseful for enabling the various embodiments of the present invention,such as the embodiment disclosed in FIG. 3, it is not intended to limitthese embodiments and the present invention taught by these embodiments.One of ordinary skill in the art after having the benefit of the hereindisclosure would readily recognize that other relationships may be usedto practice the present invention without undue experimentation.

Turning now to FIG. 4, a resonant converter 160 that attenuates orreduces spurious oscillations is disclosed in accordance with yetanother embodiment of the present invention. Resonant converter 160 mayhave a circuit structure similar to that of resonant converter 84 inFIG. 3A, and includes a switching circuit 162, a capacitor 164, a biasresistor 166, a commutation circuit 167 and a load 212 that may havesubstantially the same structure and function as switching circuit 130,capacitor 126, bias resistor 128, commutation circuit 116 and load 110,respectively, disclosed in FIG. 3A. In addition, resonant converter 160may be used as part of a power supply 170 for a static neutralizer 172.Power supply 170 may include an isolation circuit 173 and a DC powersupply 174 that provides excitation energy, such as low voltage 176 andcurrent 178, to resonant converter 160. Power supply 170, staticneutralizer 172 and isolation circuit 173 may have substantially thesame structure and function as high voltage power supply 82, staticneutralizer 80 and isolation circuit 102 in FIG. 3A.

Resonant converter 160 also includes a sensing circuit 180, aninductance set 182 and a transformer 184. Unlike the circuit structuredisclosed for resonant converter 84 in FIG. 3, sensing circuit 180 maybe implemented by using a feedback winding 186 that is wound on a core188 that is separate from a core 190 that is used by a primary winding192 of transformer 184. Besides core 190 and primary winding 192,transformer 184 also includes a secondary winding 194. Inductance set182 includes at least one inductive element formed by main winding 196on core 188 of feedback transformer 198. As shown in FIG. 4, inductanceset 182 may include main winding 196 of feedback transformer 198, and itmay also include one or more additional inductive elements, such asinductive element 200, inductive element 202 or both. Including theseadditional inductive elements reduces the energy storage requirements oftransformer 184 and feedback transformer 198, leading to cost andefficiency improvements in resonant converter 160. In one embodiment ofthe present invention, inductive elements 200 and 202 may each beimplemented using inductors that each have inductance values within anapproximate range from 170 to 1000 mH but these values are not intendedto be limiting in any way and are provided as an example implementationof a resonant converter that can drive a load within an approximaterange from 70 to 100 pF at a resonant frequency within an approximaterange from 10 to 20 kHz, assuming that resonant converter 160 includestransformer 184, feedback transformer 198, inductive element 200, andinductive element 202 as shown in FIG. 4 and that these are all wound oncores of similar size.

An end of secondary winding 194, such as end 204, connects to ground206, while the other end of secondary winding 194, such as end 208,connects to one end of inductance set 182, such as end 210. The otherend of inductance set 182, such as end 211, ultimately connects toground 206 through load 212. Secondary winding 194, inductance set 182and load 212 form a secondary tank circuit 214. Like resonant converter84 in FIG. 3A, resonant converter 160 oscillates at the resonantfrequency of its secondary tank circuit, which in this example issecondary tank circuit 214.

During operation of resonant converter 160, feedback transformer 198senses the resonant circulating current flowing through secondary tankcircuit 214 and converts this current into a differential voltage thatleads the circulating current by 90 degrees but is in phase with thevoltage across load 212, such as the voltage amplitude of outputwaveform 216. Because the current flowing through secondary tank circuit214 is approximately sinusoidal, the voltage amplitude of feedbacksignal 218 appearing across feedback winding 186 is also approximatelysinusoidal. Feedback signal 218 is used by commutation circuit 167 tocontrol switch elements 220 and 222, which may be implemented usingbipolar NPN transistors 224 and 226, of switching circuit 162. Apartfrom the implementation that generates feedback signal 218, resonantconverter 160 operates in a similar manner to that of resonant converter84 in FIG. 3A, and component selection procedures that are similar tothe procedures disclosed above can be used for both embodiments.

FIG. 5 illustrates another embodiment of the present invention thatincludes a resonant converter 240 that attenuates or reduces spuriousoscillations. Resonant converter 240 may have a circuit structure thatis similar to that of resonant converter 160 in FIG. 4, and includes aswitching circuit 242, a capacitor 244, a bias resistor 246, a load 248,a commutation circuit 249 and a transformer 250 that may havesubstantially the same structure and function as switching circuit 162,capacitor 164, bias resistor 166, load 212, commutation circuit 167 andtransformer 184 disclosed in FIG. 4. In addition, resonant converter 240may be used as part of a power supply 252 for a static neutralizer 254.Power supply 252 may include an isolation circuit 256 and a DC powersupply 258 that provides excitation energy, such as low voltage 260 andcurrent 262, to resonant converter 240. Power supply 252, staticneutralizer 254 and isolation circuit 256 may have substantially thesame structure and function as power supply 170, static neutralizer 172and isolation circuit 173 disclosed in FIG. 4.

Resonant converter 240 also includes a sensing circuit 264, aninductance set 266 and a feedback transformer 268. Sensing circuit 264may be implemented by using a feedback winding 270 that is wound on acore 272 that is separate from a core 274 that is used by a primarywinding 276 of transformer 250. Besides core 274 and primary winding276, transformer 250 also includes a secondary winding 278.

Inductance set 266 may have any number of inductive elements, includingno elements and thus, inductance set 266 may be omitted from secondarytank circuit 292. However, in the example shown, inductance set 266 maybe configured to include inductive element 280 and inductive element282. Including inductive elements in inductance set 266, such asinductive elements 280 and 282, reduces the energy storage requirementsof transformer 250 and feedback transformer 268, leading to size,volume, shape, cost, and efficiency improvements that would otherwise bedifficult to achieve by using conventional implementations. In theembodiment shown in FIG. 5, inductive elements 280 and 282 may each haveinductance values that approximately range from 170 to 1000 mH. Thesevalues are not intended to be limiting in any way and are provided as anexample implementation of a resonant converter that can drive a loadwithin an approximate range from 70 to 100 pF at a resonant frequencywithin an approximate range from 10 to 20 kHz, assuming that resonantconverter 160 includes transformer 184, feedback transformer 198,inductive element 200, and inductive element 202 as shown in FIG. 4 andthat these are all wound on cores of similar size.

Unlike in resonant converter 160, an end of secondary winding 278, suchas end 284, ultimately connects to ground 286 through a main winding 288of feedback transformer 268, while the other end of secondary winding278, such as end 290, connects to one end of inductance set 266, such asend 293. The other end of inductance set 266, such as end 295,ultimately connects to ground 286 through load 248. If inductance set266 contains no inductive elements, then end 295 and end 293 bothrepresent the same circuit node. Secondary winding 278, inductance set266, load 248 and main winding 288 form a secondary tank circuit 292.Resonant converter 240 oscillates at the resonant frequency of itssecondary tank circuit, which in this example is secondary tank circuit292.

A small core and bobbin for feedback transformer 268 may be used so thatthe inductance of main winding 288 is small and a relatively smallvoltage is produced across main winding 288 by the circulating currentsin the secondary tank circuit 292 during operation of resonant converter240. The step down ratio between feedback winding 270 and main winding288 is selected so that the resistance of bias resistor 246 reflectedacross feedback transformer 268 to main winding 288 is large compared tothe inductive reactance of main winding 288 at the operating frequencyof resonant converter 240. Selection of a relatively small value for theinductance of main winding 288 reduces the electrical potential of core274 and secondary winding 278, reducing dielectric losses in transformer250 caused by the electrical potential difference of primary winding276, secondary winding 278, and core 274.

During operation of resonant converter 240, feedback transformer 268senses the resonant circulating current flowing through secondary tankcircuit 292 and converts this current into a differential voltage, suchas feedback signal 294. Because the current flowing through secondarytank circuit 292 is approximately sinusoidal, the voltage amplitude offeedback signal 294 that appears across feedback winding 270 is alsoapproximately sinusoidal. Commutation circuit 249 receives feedbacksignal 294 and uses it to control switch elements 296 and 298, such asbipolar NPN transistors 300 and 302, in switching circuit 242. Apartfrom the implementation that generates feedback signal 294 and theconfiguration of inductance set 266, resonant converter 240 operates ina manner similar to resonant converter 160 in FIG. 4, and componentselection procedures that are similar to the procedures disclosed abovecan be used for the embodiment in FIG. 5.

Referring now to FIG. 6, another example of a resonant converter 310 isdisclosed in accordance with another embodiment of the presentinvention. Resonant converter 310 may include a transformer 312, afeedback transformer 314, a capacitor 315, an inductance set 316, a load318 and secondary tank circuit 319 that may be disposed to havesubstantially the same structure and function as transformer 250,feedback transformer 268, capacitor 244, inductance set 266, load 248and secondary tank circuit 292 in FIG. 5, except switching circuit 320includes an electronic zero-voltage switching (ZVS) control circuit 322that is coupled to switch elements 324 and 326, such as N-channelMOSFETs 328 and 330, and to both ends of feedback winding 332 offeedback transformer 314. ZVS control circuit 322 receives a feedbacksignal 334, which is in differential form, from feedback winding 332. Inresponse to feedback signal 334, ZVS control circuit 322 generatescontrol signals that are suitable for controlling switch elements 324and 326 so that these switch elements commutate, creating andmaintaining an oscillation in secondary tank circuit 319. Thisoscillation flowing through secondary tank circuit 319 results in a highvoltage output waveform 336, such as a waveform suitable for generatingions by corona discharge when applied to an emitter (not shown).

The ZVS control circuit 322 has a push-pull circuit configuration. ZVScontrol circuits, including ZVS control circuits that have a push-pullcircuit configuration, are generally known, and may include anintegrated ZVS controller IC along with additional power supply andsupport components, which are not illustrated in FIG. 6 to avoidovercomplicating the herein disclosure. In the absence of an acceptablecommercially available ZVS control IC for a given application, a customASIC or a combination of programmable logic and other electroniccircuitry can be used to obtain the desired control features describedherein. In addition, although switch elements 324 and 326 can beimplemented as N-channel MOSFETs, such as MOSFETs 328 and 330, one ofordinary skill in the art would readily recognize after receiving thebenefit of the herein disclosure that other types of electronic switchesmay also be used. These might include P-channel MOSFETs, JFETs, NPN orPNP bipolar transistors, IGBTs, MCTs, thyristors, mechanical relays,solid-state relays, or the like.

Resonant converter 310 may be used with a static neutralizer 338 anddisposed to receive excitation energy, such as low voltage 340 andcurrent 341, from a DC power supply 342. An isolation circuit 344 mayalso be employed to isolate DC power supply 342 from resonant converter310. Static neutralizer 338, DC power supply 342 and isolation circuit344 may have substantially the same form and function as staticneutralizer 254, DC power supply 258 and isolation circuit 256previously disclosed above with respect to FIG. 5.

FIG. 7 includes an illustration of a resonant converter 350 inaccordance with yet another embodiment of the present invention.Resonant converter 350 is substantially similar to resonant converter310 previously disclosed above, except it includes a sensing circuit 352that provides a single-ended feedback signal 354 and a ZVS controlcircuit 356 configured to accept a single-ended feedback signal, such assingle-ended feedback signal 354. Sensing circuit 352 may be implementedusing a circuit that provides a feedback signal in the form of avoltage, such as a capacitive divider 358. Capacitive divider 358includes a high voltage capacitor 360 having an end 362 coupled inseries with a low-voltage capacitor 364. The coupling between highvoltage capacitor 360 and low voltage capacitor 364 forms an outputterminal 365 that is coupled to ZVS control circuit 356, providing asignal path for single-ended feedback signal 354. The other end 366 ofhigh voltage capacitor 360 is coupled to an element within secondarytank circuit 367 that reflects a voltage when a current oscillates insecondary tank circuit 367, such as a load 368 that will be driven byresonant converter 350. If load 368 is an emitter (not shown), such asan emitter used in a static neutralizer 370, and static neutralizer 370includes an ion balance circuit 372, then end 366 may connect to eitherside of ion balancing circuit 372. The end of low-voltage capacitor 364that is not coupled to high voltage capacitor 360 is coupled to ground374.

In one implementation example, high voltage capacitor 360 may beimplemented using copper traces on a printed circuit board (not shown).High voltage capacitor 360 may have a value that is typically muchsmaller than the capacitance of load 368, enabling the voltage sensingfunction of capacitor 360 to be carried out without adding significantloading to resonant converter 350. Low voltage capacitor 364 may beselected to have a much larger capacitance value than high voltagecapacitor 360, allowing capacitor 364 divide down the output voltage toa level that is appropriate for the circuitry in ZVS control circuit356. In the example shown, high voltage capacitor 360 and low voltagecapacitor 364 may have values within an approximate range from 0.1 to2.0 pF and 0.2 to 4.0 nF, respectively. Ion balance circuit 372 may havesubstantially the same structure and function as ion balance circuit 26in FIG. 2.

In accordance with yet another embodiment of the present invention, aresonant converter 380 is illustrated in FIG. 8. Resonant converter 380is similar to resonant converter 310 previously disclosed above withreference to FIG. 6. Resonant converter 380 includes a feedbacktransformer 382, an inductance set 384, a load 386 and a secondary tankcircuit 388, which may be substantially similar in structure andfunction to feedback transformer 314, inductance set 316, load 318 andsecondary tank circuit 319 in FIG. 6. Resonant converter 380 alsoincludes a transformer 396 and a commutation circuit 390, which includesa switching circuit 392, a control circuit 394, and a capacitor 398.Switching circuit 392 is disposed to receive excitation energy, such aslow voltage 400 and current 401, from a DC power supply 402. Anisolation circuit 404 may be utilized to isolate transformer 396 from DCpower supply 402 in order to prevent DC power supply 402 from acting asa load on resonant converter 380. Isolation circuit 404 may havesubstantially the same structure and function disclosed for isolationcircuit 344 in FIG. 6.

Switching circuit 392 includes a transistor H-bridge formed from fourswitch elements, such as n-channel MOSFETs 408, 410, 412 and 414. Theuse of n-channel MOSFETs in switching circuit 392 is not intended to belimiting in any way but any type of switch element may be used that iscompatible with the control circuit 394. Transformer 396 is similar totransformer 312 except primary winding 416 does not include acenter-tap. In addition, primary winding 416 may be configured toinclude approximately half the number of turns used for the primarywinding in transformer 312. Reducing the number of turns in primarywinding 416 lowers its inductance may require capacitor 398 to have acapacitance that is approximately four times the value of capacitor 315in FIG. 6.

Control circuit 394 may be implemented by using a ZVS H-Bridge controlcircuit, which is widely known and available. Control circuit 394alternately activates transistors 408 and 414 or transistors 410 and412, which reverses current through primary winding 416 each timefeedback signal 418 crosses a selected threshold, such as zero volts.Feedback signal 418 is generated by feedback winding 422 and is indifferential form. This reversing current in primary winding 416 excitessecondary tank circuit 388, generating a high voltage output waveform424.

While the present invention has been described in particularembodiments, it should be appreciated that the present invention shouldnot be construed as limited by such embodiments. Rather, the presentinvention should be construed according to the claims below.

1. A high voltage power supply for a static neutralizer, comprising: aload that includes an emitter module having an emitter, referenceelectrode and a capacitance value; a resonant converter having aresonant frequency and an output electrically coupled to said load, saidresonant converter for generating an output waveform at said output thathas an amplitude sufficient for generating to ions by corona dischargewhen said load receives said output waveform; and wherein said load ispredominantly capacitive when said resonant converter is operating atsaid resonant frequency.
 2. The high voltage power supply of claim 1,wherein said load further includes an ion balance circuit that iscoupled between said emitter and said output.
 3. The high voltage powersupply of claim 1, wherein said resonant converter further includes: atransformer that includes a primary winding and a secondary winding; acommutation circuit having a switching circuit; a sensing circuit forgenerating a feedback signal; and wherein said commutation circuitdelivers energy from a DC power supply through an isolation circuit tosaid primary winding and uses said feedback signal to maintain anoscillation within said resonant converter.
 4. The resonant converter ofclaim 3, wherein said secondary winding and said load have a totalinductance equal to:$L_{R} = \frac{1}{\left( {2\pi \; F} \right)^{2}C_{R}}$ whereinL_(R) is the total inductance of said secondary winding and said load,C_(R) is the capacitance of said load, and F is a desired operatingfrequency of the resonant converter.
 5. The high voltage power supply ofclaim 3, wherein said isolation circuit includes an AC choke coil. 6.The high voltage power supply of claim 5, wherein said load furtherincludes an ion balance circuit, said ion balance circuit including acapacitor with a capacitance value; and wherein said capacitance valueof said capacitor is at least four times greater than said capacitancevalue of said emitter module.
 7. A resonant converter for use with apower supply of a static neutralizer, said static neutralizer includinga waveform output and an emitter module having at least one emitter anda reference electrode; said power supply including a DC power supplyhaving an output, said resonant converter comprising: a transformer thatincludes a primary winding, a secondary winding, and a core; acommutation circuit coupled to said transformer and for receiving acurrent from said output of said DC power supply; a load having a firstend coupled to the waveform output and including the emitter module; asensing circuit that generates a feedback signal, said feedback signalprovided to said commutation circuit; and wherein said load ispredominantly capacitive during operation of said resonant converter. 8.The resonant converter of claim 7, wherein excitation energy is coupledfrom said output through an isolation circuit to said transformer. 9.The resonant converter of claim 8, wherein said isolation circuit iscoupled between the output and a first leg of said primary winding. 10.The resonant converter of claim 7: wherein said secondary windingincludes a first secondary winding end; and further including aninductance set coupled between said first secondary winding end and thewaveform output.
 11. The resonant converter of claim 7, wherein saidsecondary winding includes a first secondary winding end and a secondsecondary winding end that are respectively coupled to the waveformoutput and a second end of said load.
 12. The resonant converter ofclaim 7, further including an inductance set that includes at least oneinductive element connected in series with said secondary winding. 13.The resonant converter of claim 12, wherein one of said at least oneinductive element includes a main winding on a second transformer. 14.The resonant converter of claim 12, wherein said sensing circuitincludes a feedback winding on said second transformer.
 15. The resonantconverter of claim 12, wherein said secondary winding is coupled to oneend of said inductor set.
 16. The resonant converter of claim 12,wherein said secondary winding, said inductance set and said load arepart of a secondary tank circuit that has a total inductance equal to:$L_{R} = \frac{1}{\left( {2\pi \; F} \right)^{2}C_{R}}$ whereinL_(R) is the total inductance of said secondary tank circuit, C_(R) isthe capacitance of said load, and F is a desired operating frequency ofthe resonant converter.
 17. The resonant converter of claim 12, whereinsaid commutation circuit allows excitation energy to be transmitted fromsaid DC power supply to said transformer through said primary winding.18. The resonant converter of claim 17, wherein: said primary windingfurther includes a second leg and an end from said first leg and saidsecond leg provides a center tap; and said switching circuit includestwo switch elements for switching current between said first and saidsecond legs, and a biasing element coupled between one of said switchelements and the output.
 19. The resonant converter of claim 18, whereinsaid switching circuit further includes a capacitor for absorbingtransients from said primary winding during operation of saidcommutating circuit.
 20. The resonant converter of claim 18, whereinsaid biasing element includes a resistor.
 21. The resonant converter of17, wherein said commutation circuit further includes a control circuitthat controls said switching circuit in response to said feedbacksignal.
 22. The resonant converter of claim 7, wherein said sensingcircuit includes a feedback winding on said transformer.
 23. Theresonant converter of claim 7, further including an inductance set thatincludes a first inductive element and a second inductive element; andwherein said secondary winding is coupled in series between said firstand second inductive elements.
 24. The resonant converter of claim 23,further including a second transformer; and wherein said sensing circuitincludes a feedback winding on said second transformer, and wherein saidfirst inductive element includes a main winding on said secondtransformer, said main winding having an end coupled to said secondarywinding.
 25. The resonant converter of claim 23, wherein said sensingcircuit includes a feedback winding on a second transformer, and saidfirst inductive element includes a main winding on said secondtransformer.
 26. The resonant converter of claim 7, wherein said loadincludes an ion balance circuit.
 27. The resonant converter of claim 26,wherein said commutation circuit includes a switching circuit thatincludes at least one switch element.
 28. The resonant converter ofclaim 27, wherein said switch element is a bipolar transistor.
 29. Theresonant converter of claim 27, wherein said at least one switch elementis an element obtained from a group consisting of a MOSFET, JFET, IGBT,MCT, thyristor, opto-isolator, electromechanical relay and solid-staterelay.
 30. The resonant converter of claim 27, wherein said controlcircuit includes a ZVS push-pull control circuit.
 31. The resonantconverter of claim 27, wherein said control circuit includes a ZVSH-Bridge control circuit.
 32. The resonant converter of claim 7, whereinsaid primary winding includes a first leg and a second leg; and whereinaid commutation circuit includes a capacitor having one end coupled tosaid first leg and another end coupled to said second leg.
 33. Theresonant converter of claim 32, wherein said coupling of said capacitorto said first and second legs enables said capacitor to absorbtransients from said primary winding during operation of saidcommutating circuit.
 34. The resonant converter of claim 7, wherein saidisolation circuit is coupled between the output and a first leg of saidprimary winding, and said isolation circuit includes an AC choke coil.35. The resonant converter of claim 7, wherein said sensing circuitincludes a capacitive divider.
 36. The resonant converter of claim 7,wherein said primary winding includes a first leg and a second leg andsaid commutation circuit includes switch elements for switching currentalternately between said first and said second legs.
 37. The resonantconvert of claim 36, wherein said commutation circuit includes a controlcircuit disposed to receive said feedback signal from said sensingcircuit, and to generate control signals for controlling said switchelements so that said switch elements switch said current alternatelybetweens said first and second legs.